The use of pressure building coils interposed between the walls of a double-walled cryogenic tank is well known in the art. Such coils are conventionally placed such that low temperature fluid passes from the tank directly into the coil. The coil is in contact with the outer wall of the tank, and heat transferred there vaporizes the gas. Pressure building coils are conventionally in the form of tubes which are helically disposed around the interior of the outer wall of a double-walled tank.
Pressure building coils of the type described above are used to maintain gas pressure in the tank head as gas is withdrawn. Conventionally, the pressure building coil feeds directly into the top of the tank through a regulator which permits gas from the pressure building coil to be added to the tank when the head pressure above the liquid level in the tank falls below a set value.
Conventional systems, however, are deficient in several respects. First, as the low temperature fluid is withdrawn through the bottom of the tank, impurities which may be settled in or near the bottom of the tank are also withdrawn. These impurities may be deleterious to the proper function of the pressure building coil, the gas use line, or may be otherwise unsuitable for passage through the pressure building coil and into said gas use line. Use of the presure bulding coil may be disrupted as a result of the presence of these impurities. Thus, it is desirable to provide a means by which these impurities may be removed from the low temperature fluid prior to entry into the pressure building coil.
The conventional system described above also has the problem of pressure head collapse when demand for gas from the tank becomes too great. Thus, if gas is withdrawn from the tank at a high rate, the pressure head may be lowered to an unacceptable value. The pressure building coil is unable to compensate for this pressure drop, since the rate of pressure increase through the pressure building coil is typically less than the rate of pressure loss resulting from withdrawal of gas through the gas use line. It is therefore desirable to provide a system in which the pressure in the gas use line may be increased during periods of high demand, the pressure increase being derived directly from the pressure building coil.